Most cooling systems of electronic devices, such as chips in laptop or desktop computers, use forced convection of air associated with large radiator surfaces to remove the heat dissipated and to decrease the operating temperature of the processor. The clock speed of a processor, its reliability and lifetime are strongly dependent on these temperatures. With high heat flux sources and/or when the volume available is small, forced convection of air is not sufficient to remove large quantities of heat. New processors exhibit heat fluxes higher than 100 W/cm2. For these applications, more efficient cooling systems are based on the circulation of a liquid, as disclosed for example in Phillips et al. U.S. Pat. No. 4,894,709, or in Bonde et al. U.S. Pat. No. 5,099,311. Some cooling systems are based on the vaporization of a cooling liquid inside a microstructure, in close contact with the heat source. Such cooling systems are disclosed for example in Tonkovich et al. U.S. Pat. No. 6,200,536, or in Parish et al. WO 02/080270 A1. A further example of a micro-refrigeration system comprising an evaporator and a condenser based on micro-channel heat exchange elements is disclosed by Beebe et al. in U.S. Pat. No. 6,148,635.
This concept for heat removal is based on evaporation of fluids in microchannels. Some authors propose a somewhat arbitrary classification into microchannels, having internal diameters of between 50 μm and 600 μm, and minichannels having diameters of between 600 μm and 3 mm. The inventors consider that these figures do not constitute absolute thresholds, and that according to a functional threshold criterion, a microchannel is a small diameter channel in which the vapor bubbles are confined by the size of the channel and grow primarily in length once their cross-sectional diameter have nearly reached the internal diameter of the channel. Typically, this occurs indeed in channels smaller than about 2–3 mm in size. A microchannel may be round, square, triangular or of any other cross-sectional geometrical shape, and straight in length or have a complex loop geometry. A microchannel element may consist of one, usually meandering microchannel, or of several or many microchannels arranged side by side. The microchannel element may be one integral piece or may be an assembly of individual tubes into one multichannel element. This element may be arranged in thermally conductive contact with the device to be cooled. Alternatively, the microchannel(s) may be integrated directly into the device to be cooled.
Due to the presence of small cavities in the wall of the microchannel(s) and the high wetting capability of the working fluids envisaged in such micro-evaporators, the liquid superheat ΔTsat,i associated with the initiation of boiling is quite high, up to 50° C. Thus, during transient behavior, this temperature overshoot is a penalizing phenomenon that creates thermal stresses and obligates electronic component manufacturers to reduce the performance of the chips in order to reduce this temperature peak that precedes the efficient heat transfer by phase change.
Honda and Wei (Exp. Thermal and Fluid Science, 28, 159–169, 2004) have summarized the challenges that must be solved to use a two-phase micro-heat exchanger for such applications: (i) mitigation of the wall temperature overshoot at boiling incipience, (ii) enhancement of the nucleate boiling heat transfer coefficient, (iii) increase of the critical heat flux (CHF). For pool boiling, Honda and Wei noticed that surface enhancement permits to lower the boiling incipience superheat but does not work if subcooled conditions are reached because in this case the liquid floods the microcavities. The most effective way to reduce ΔTsat,i is then to use a liquid completely saturated with a incondensable gas as coolant, but this pool boiling solution is not adaptable in microchannel flows because pure fluid is needed in the loop and because it is also difficult to implement surface treatment without increasing roughness. Until now, no reliable solution has been found to this problem inside small channels.
The present invention proposes a solution to solve these problems, in particular the temperature overshoot at boiling incipience.